Electrochemistry of Chromium-Metalloid Alloys in Sulfuric Acid—II

Abstract

The electrochemistry of glassy Cr72P28, Cr87P13, Cr70B30, and mixed–phase glassy ~A–15, nanocrystalline Cr93P7 were examined in comparison to chromium. For oxidizing conditions these materials are passive and exhibit similar film formation kinetics. In reducing environments, chromium is in the active state. The excellent corrosion behavior of Cr1–xPx and Cr70B30 is attributed to the fact that, unlike chromium, these materials cannot be stably activated. The i–E characteristics of chromium–metalloid materials resemble that of chromium–noble metal alloys. By analogy to the noble metals, alloying with phosphorus and boron accelerates the kinetics of proton reduction such that the steady–state electrochemical potential of the system is set in a domain consistent with the passive state of chromium. The metalloids also block the metal dissolution reaction at active potentials. For Cr1–xPx the degree of anodic inhibition increases significantly with phosphorus concentration. The remarkable effect of the metalloids on corrosion resistance may be partially attributed to the metal–loid–hydride and proton reduction equilibria being positive of the active–passive transition of chromium. This correlation helps rationalize the varying effects of alloying boron and phosphorus with different transition metals, e.g. reactive Fe80(B, P)20 vs. passive Cr80(B, P)20. © 1991, The Electrochemical Society, Inc. All rights reserved.